The techniques of stop-flow measurements of turbidity, fluorescence and proton release and quench flow measurements of phosphate formed in ATP hydrolysis are being used to study the mechanism of actomyosin ATPase and its regulation by Ca. The rate constants for the various intermediate stops in myosin ATPase have been determined at pH 8 and 20 degrees. These studies are being extended to various pH's and temperatures. The steps in actomyosin ATPase from the formation of the actomyosin-products complex to product release are being investigated. The finding that the two heads of myosin are not equivalent in the actomyosin complex will be the basis of studies of head interaction in the mechanochemical step.